BACKGROUND OF THE DISCLOSURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] The present application claims the benefit of U.S. Provisional Patent Application Serial No. 62/906,843 filed September 27, 2019, which is fully incorporated herein by reference.

Field of the Disclosure

[002] The disclosure relates to multiple fiber lasers and amplifiers integrated with a photonic integrated circuit (PIC).

Background of the Disclosure

[003] The silicon photonic platform can be made with extremely high yield because of the nearideal starting wafers and the maturity of silicon processes developed for electronics. These desirable features have led to the success of silicon photonics broadly implemented in industrial and scientific applications. The spectroscopic measurements, wavelength division multiplexing (WDM), frequency modulated light detection and ranging (LIDAR), optical coherence tomography (OCT), biomedical and, of course, telecommunication are just a few fields greatly benefiting from the use of silicon photonics. The popularity of silicon (Si) is readily explained by its natural abundance and transparency to the electromagnetic field in the O- and C- bands (1230 - 1260 nm and 1530 to 1565 nm, respectively.)

[004] Silicon photonics, for all its benefits with functionality and yield, still lacks a monolithically integrated light source. Silicon and related group IV materials, Such as Ge, are indirect bandgap semiconductors, and thus it is very difficult to create an efficient light source on silicon. In recent years the lasers, using silicon photonics technology, have attracted considerable interest due to the potential low-cost fabrication in CMOS foundries, borrowed from the microelectronics industry. Their integration/co-packaging with a PIC can lead to a total silicon photonics solution for future low-cost and small form factor coherent modules. The PIC systems include SiO ₂ PIC or Planar Lightwave Circuits (PLCs), silicon-on-insulator also referred to as silicon photonics (SiP), lithium niobate (LiNbOa), and III-V PICs, such as InP and GaAs. While silicon photonic lasers can be a cost- and space-efficient solution, their commercialization has not been successful due to a few limitations. [005] First, silicon photonics devices suffer from large on/off-chip optical coupling loss. The resultant laser output power is not sufficient to compensate for the large insertion loss PIC and modulation loss if a modulator is used. Secondly, the propagation loss of silicon waveguide is much greater than that among free-space optical components and other material systems such as silica or silicon nitride. This prevents the use of long external cavities to generate more ‘pure’ and lower noise lasing light, which is a key enabler for high-order modulation formats to carry more information. Thirdly, silicon material is very sensitive to the thermal disturbances, for example, from package temperature changes or gain medium current changes. As a result, cost-efficient and technically viable integration of high-performance light sources into SiP circuits remains a challenge. The above and other limitations explain why silicon photonics still lacks a monolithically integrated light source.

[006] Yet several approaches have been proposed to integrate gain medium onto a silicon photonic circuit such as the III-V material coupled via lenses or free space. Unfortunately, this approach requires a sealed package known as the gold box which renders the packages cost inefficient. Another approach is to integrate the III- V material via edge coupling, vertical coupling, bonding or direct hetero-epitaxy of these materials on Si. All of the above-disclosed approaches can be compatible with non-hermetic packages, but the use of the III- V material introduces the known limitations including a patterning effect for amplification and high temperature sensitivity. Additionally, if polarization insensitive devices are required, and they usually are, the saturation power is reduced limiting thus the output power.

[007] Fairly recently, attempts have been made to integrate erbium-doped waveguide amplifiers (EDWA) on a PIC, but the performance of these photonic devices is far from the standards required by commercial applications. Yet the integration of EDWAs on silicon is very promising. Indeed, coherent communications have revolutionized core networks and are expected to take over a large share of the market in metropolitan and inter-datacenter networks in the very near future. As modulation formats are moving to higher order of constellation, narrow linewidth lasers with the external cavity are simply irreplaceable. However, as mentioned above, the problem with the known integration methods remains, i.e., the laser cavity length is defined by a PIC and therefore has a limited length due to a small footprint and loss limitations.

[008] A close relative of EDWA is erbium-doped fiber gain media. Generally, fiber to PIC coupling methods are known and include edge-coupling, grating-coupling, and most recently evanescent-coupling — each having their own performance advantages and limitations. The fiber- based gain medium on silicon offers longer cavities reducing the laser linewidth, provides an efficient amplification, has no patterning effect, minimizes temperature sensitivity and increases high output power. Yet the fiber gain media have been only used for laser generation with discrete devices such as opto-VSLI-processors, isolators with a silicon photonics (SiP) micro-ring, WDM couplers and polarization controller, SiP Bragg grating with WDM coupler, optical circulator and an erbium doped fiber (EDF) all located within a fiber loop outside the PIC. The abundance of optical passive and active elements within the fiber loop increases the footprint of a photonic device.

[009] One of the reasons for slow developments of integrated fiber lasers and amplifiers is the necessity of pumping fiber-based gain media by respective dedicated pumps each of which is typically configured as an off-board diode laser. The pumps add cost and complexity to hybrid photonic laser systems which is, of course, a strong deterrent to the use of integrated EDFAs. [010] A need therefore exists for a broad application of the fiber-based gain media integrated in various photonic circuits and allowing for high performance and multiple functionalities on a single PIC.

[Oil] Another need exists for multiple fiber gain media on a single PIC sharing a single pump.

SUMMARY OF THE DISCLOSURE

[012] The invention illustrated by the disclosed structural examples satisfies these needs. The examples are structurally and functionally interrelated and, as will be more apparent from the following description, can be combined with one another or used individually without contradicting the inventive concept.

[013] The inventive concept provides for the integration of a fiber-based gain medium and a PIC as components of a non-hermetic photonic device. Provided with at least one fiber gain medium, the PIC of the inventive photonic device is integrated with Si photonic passive and active elements, while a fiber link between the gain medium and PIC is substantially free from these elements. The photonic device is characterized by a high performance including a narrow linewidth, high power tunable laser with no temperature dependence and no patterning effects and high saturation power for a high output power of the amplifier section. [014] Accordingly, the inventive hybrid photonic device is configured with a PIC, one or multiple gain media or active fibers (i.e., fibers doped with light emitters such as ions of rare earth elements) coupled to the PIC, and at least one external pump common to at least some of all active fibers. The inventive photonic device may have a simple resonant cavity with practically limitless on-chip capability in accordance with any given task at hand and is characterized by a low power consumption and low cost. In case of multiple fiber-based gain media, the number of pumps in the inventive configuration varies between one (1) and, wherein n is at least one less than the number of active fibers. If the situation requires that each gain medium is energized by a designated pump, then this invention does not exclude such a configuration. Preferably, but not necessarily, the pump includes a diode laser consuming no more than a few mW. Alternatively, the pump may be configured, for example, as a fiber laser or have any other suitable configuration.

[015] In accordance with one example of the inventive photonic circuit, the gain media include at least one active fiber having one or both fiber ends coupled to the PIC. The active fiber and the PIC form a hybrid resonant cavity therebetween defined between a pair of spaced reflectors. The reflectors may be provided in the fiber or fiber and chip, respectively or only in the chip and spaced apart to define therebetween an extended resonant cavity. Preferably, all fiber gain mediums are in direct optical communication with the PIC forming thus respective fiber links all free of active and passive photonic elements, which in turn, are integrated in the PIC. However, as known to one of ordinary skill, a fiber-based gain medium may be configured with a pair of input and output transverse single mode fibers spliced to respective opposite ends of the active fiber with a free end of the output passive fiber being optically coupled to the PIC. The configuration of the reflectors may include fiber Bragg gratings, fiber ring mirrors, mirrors integrated in the PIC, Sagnac loop mirrors and others known in the art. Preferably, the laser is tunable.

[016] The exemplary photonic circuit is further configured with at least one or more fiber amplifiers coupled to the PIC and operative to amplify respective input and output light signals. Similar to the gain medium forming a hybrid resonant cavity with the PIC, all fiber amplifiers define respective fiber links free active and passive photonic device which are thus integrated with the PIC. A spatial filter may be added externally to the PIC or internally onto it to filter out the desired wavelength of the output light signal. [017] The pump light emitted by a single pump can be coupled into the fiber laser and fiber amplifiers in accordance with several pumping schemes. For example, the pump light can be directly injected into all active fibers outside the PIC. Alternatively, pump light can be coupled into the active fibers through the PIC.

BRIEF DESCRIPTION OF THE DRAWINGS

[018] The above and other examples, features and functions of the inventive PIC will become more readily apparent from the specific description illustrated by the following drawings:

[019] FIG. 1 illustrates the disclosed hybrid photonic device;

[020] FIG. 2 is an exemplary optical schematic of the photonic device of FIG. 1 ;

[021 ] FIG. 3 is another exemplary optical schematic of the photonic device of FIG. 1 ;

[022] FIG. 4 is an optical schematic of the transceiver based on the disclosed concept;

[023] FIGs. 5 and 6 are respective optical schematics of a transmitter based on the disclosed concept;

[024] FIG. 7 is an optical schematic of the receiver based on the disclosed concept; and [025] FIGs. 8 - 11 are respective exemplary optical schematics of a resonant cavity utilized in the disclosed photonic device.

SPECIFIC DESCRIPTION

[026] The following description provides an illustration and a further understanding of the inventive concept, but is not intended as a definition of the limits of the present disclosure. The following disclosure, together with the drawings, serve to explain principles and operations of the described inventive concept. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure.

[027] FIG. 1 diagrammatically illustrates the inventive photonic device 10 including at least one fiber gain medium 14 ₁ , which is the active fiber that has at least one core doped with ions of light emitters, and a PIC 12. Any of or all gain media may be configured as part of a hybrid laser 14 ₁ , as shown in FIG. 1, and/or one or more fiber amplifiers 14 ₂ . In case of the laser, a hybrid resonant cavity is formed between gain medium 14 ₁ and PIC 12. In contrast to the prior art, the active fiber extending to PIC 12 has no photonic elements coupled thereto. In other words, the active fiber is free from photonic elements. The photonic elements may include active elements, such as polarization modules, photodiodes and optical modulators and other devices having an intended dynamic interaction between light and matter, and photonic passive elements, such as reflectors, isolators, couplers, attenuators, circulators and other known elements. Some of the passive elements can be characterized as partway between passive and active elements. All of these photonic elements are integrated in PIC 12. However, a possibility still exists to form the optical cavity in the fiber by having one or both fiber Bragg gratings (FBG) 18, 20, respectively written in the fiber. In case of one FBG, the other reflector is provided in PIC 12. If the reflector or reflectors are written in fiber, it can be advantageous to write them in a passive fiber having opposite ends thereof spliced to the output of active fiber 14 ₁ and PIC 12, respectively. [028] Depending on the configuration of the resonant cavity of gain medium 14 ₁ (provided the latter is a fiber laser), all photonic passive elements including reflectors, polarization splitters, controllers and others, also may be integrated in PIC 12. The resonant cavity of the fiber laser is defined between reflectors 18, 20 which are selected from fiber Bragg gratings (FBG) and integrated in silicon ring mirrors, distributed Bragg grating, Sagnac loop mirrors, loop mirrors and others known in the photonic art.

[029] In accordance with the inventive concept, photonic device 10 is configured with multiple gain media 14 ₁ , 14 ₂ ... and 14 _n with gain media 14 ₂ ... 14 _n each being configured either as a fiber laser or fiber amplifier. For example, amplifier 14 ₂ may be an input amplifier with input fiber 14in, whereas amplifier 14n is an output amplifier with an output fiber 14out. A photonic device of the known prior art is configured with multiple fiber lasers/amplifiers energized by respective dedicated pumps in accordance with an end- or side-pumping technique. In contrast, disclosed photonic device 10 is configured with a pump 16 optically coupled to multiple gain media. Preferably, pump 16 is a single pump exciting all three shown fiber gain media. The minimal number of pumps is application specific, but in accordance with the inventive concept, this number is always less than or equal to the number of active fibers. It may be advantageous to use a combination of fiber-based gain medium and III - V gain material 15, which is bonded to PIC 12 or located outside it, to minimize unavoidable losses in PIC 12 as signals are guided between the input and output of PIC 12.

[030] FIG. 2 illustrates an example of photonic device 10 including a tunable fiber laser 22 with a hybrid resonant cavity, which is formed between the gain medium and PIC 12, input signal and output signal fiber amplifiers 24, 26 respectively, and pump 16. The single pump 16 is directly optically coupled to the gain media of respective laser 22 and amplifiers 24, 26. After optical amplification of the output signal, a spatial filter 28 may be located downstream from output signal amplifier 26 to suppress excessive noise in the amplified output signal.

[031] FIG. 3 illustrates a configuration similar to that of FIG. 2. However, pump 16 injects pump light into fiber laser 22 and fiber amplifiers 24, 26 through PIC 12. The pitmp light can be coupled into the gain medium in a co-propagating direction with the signal light. Alternatively, the pump light can be coupled into a gain medium in a counter-propagating direction, as shown in FIG. 3, or both co- and counter-propagating directions. The gain media, as mentioned above, is doped with ions any of the known light emitters including rare earth elements, but for the descriptive purposes, all active fibers are shown to be doped with ions of erbium (Er).

[032] FIG. 4 illustrates photonic device 10 which, in this example, is a transceiver used in telecommunications. The Er-doped fiber gain medium 22 and PIC 12 form a tunable Fabry- Perot resonator therebetween which is defined between at least one or more ring filter mirrors 34 ¹ , 34 ⁿ and mirror 36 all integrated in PIC 12. The multiple ring filter mirrors 34 ¹ , 34" are instrumental in utilizing the Vernier effect, which is a well-known technique that provides for the suppression of all lasing modes except for the desired resonant longitudinal mode. Accordingly, the hybrid resonator outputs light that can be effectively tuned by a phase shifter 40, well known to one of ordinary skill.

[033] Characteristically, the illustrated transceiver receives an input signal carrying broadband light which is amplified by an input gain medium or fiber amplifier 24 prior to the injection thereof into PIC 12. Accordingly, illustrated photonic device 10 is configured with three gain media 22, 24 and 26, respectively all energized by single pump 16 injecting pump light into the gain media through three WDM couplers 30.

[034] The mixing between the amplified input signal and a tapped off portion of the output lasing light in a coherent receiver 45 produces the detection of the desired information. The output light is divided in a splitter 38, which is integrated in PIC 12 downstream from partial reflectivity mirror 36, guiding the remaining portion of the lasing light toward a transmitter also integrated in PIC 12. The transmitter includes at least one modulator 42 selected from a phase or amplitude modulator. Preferably, the transmitter is configured with multiple modulators 42 receiving respective parts of the remaining portion of the output lasing light from another splitter 32. The modulated portions are collected in a combiner 34 and amplified in output signal fiber gain medium 26. The amplified modulated output light is further filtered within PIC 12 by spatial filter 28 optionally integrated in PIC 12.

[035] FIGs. 5 and 6 illustrate a transmitter 44 configured in accordance with the inventive concept providing for multiple gain media 22, 26 which receive pump light from pump 16 common to both gain mediums. The illustrated transmitter 44 can be used as a stand-alone hybrid photonic device or incorporated in the transceiver of FIG. 4.

[036] In particular, FIG. 5 shows PIC 12 provided with the transmitter’s components including the above discussed hybrid resonator between ring filter mirror 34 ¹ ... 34 ⁿ and mirror 36, splitter 32, phase shifter 40, one or more modulators 42 and spatial filter 28 for suppressing the undesirable noise in the amplified output signal. According to the configuration of FIG. 6, the latter is not integrated in PIC 12, but located outside PIC 12 downstream from output light fiber amplifier 26.

[037] FIG. 7 exemplifies a receiver 46 provided with gain media 22 and 24 excited by pump light from pump 16 which is common to both gain media. The gain medium 22 and PIC 12 form a resonator in PIC 12 between ring filter mirrors 34 ¹ ... 34 ⁿ and mirror 36. The spatial input filter 28 also integrated in PIC 12 cuts a narrow line out from broadband input light.

[038] FIGs. 8 and 9 illustrate respective configurations of a ring resonator hybrid laser. The broadband output coupler 52 is integrated in PIC 12 and configured as a partial reflectivity mirror letting part of the light go through and another part be guided back into the cavity. It can include an adiabatic coupler and a curved coupler. One or more intra-cavity filters 48 and phase shifter 40 constitute the ring cavity and operate to tune the hybrid laser to output the desired lasing wavelength. Optionally inserted into the ring cavity is an isolator 50 controlling either a clockwise or counter-clockwise signal propagation direction in the ring cavity. Except for WDM coupler 30, all of the components mentioned above are integrated in PIC 12. The fiber of gain medium 14 ₁ is preferably polarization-maintaining for providing the desired polarization of the light signal without a need for the polarization controller.

[039] FIG. 9 shows a more specific implementation of the ring resonator hybrid laser shown in FIG. 8. Like in FIG. 8, here the ring resonator is defined between PIC 12 and the fiber and receives pump light from WDM broadband directional coupler 30 which is located between pump 16 and gain medium 14 ₁ . One or more ring filter mirrors 34 ₁ ... 34 _n along with phase shifter 40 provide the selection of the desired lasing wavelength, generated in response to coupling pump light at another pump wavelength into the resonant cavity through WDM 30, and all inserted in the ring cavity. The output coupler 52, acting as a partial reflectivity mirror, guides the generated light at the lasing wavelength from the resonant cavity.

[040] FIGs. 10 and 11 represent a linear architecture of the hybrid cavity. FIG. 10 illustrates the hybrid resonator defined between an FBG 56 written in the outside end of gain medium 14 ₁ and mirror 36 which is integrated in PIC 12. The cavity tuning to the desired lasing wavelength is provided by one or more intra-cavity filters 48 and phase shifter 40. As can be seen, pump 16 and gain medium 14 ₁ are arranged in accordance with a side pumping technique. FIG. 11 shows the hybrid resonator defined between ring filter mirrors 34 ¹ ... 34" and mirror 36.

[041] Having thus described several examples, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the examples discussed herein. Accordingly, the foregoing description and drawings are by way of example only.